Thermal Stress in Glass

As with many other materials, as glass is heated, it expands. Thermally induced stresses, or thermal stresses, are generated when one region of a piece of glass expands more than another region. In the case of glazing within a frame, the generation of thermally induced tensile stresses at the edge will result from the central region of the glass being heated relative to the edges, which may be concealed within a framing system.

Should the amount of stress generated be great enough to overcome the strength of the glass at any point on the edge, then a fracture will occur.

The initial fracture will occur perpendicular to the edge, as the glass is effectively being pulled either side of the origin. Typically the edge of the glass will be concealed within a framing system until it is removed, and so breakage due to thermally induced stresses cannot be confirmed. However, if the edge is assessed the initial fracture can typically be easily identified, as through the thickness and relative to the edge, the fracture will be at 90o, as illustrated below.

Factors influencing Thermal Stress

Beyond the basic scenario of glazing within a framing system, other factors can influence the likelihood of failure to thermal stresses, including;

•           Glass Absorption and Unit Configurations

•           Shading

•           Framing Systems

•           Backups, Blinds and Internal Obstacles

•           Internal Heating

Solar Absorptance

With the addition of SGG PLANITHERM or SGG COOL-LITE coatings to our standard SGG PLANICLEAR substrate, there will also be some inherent change to the absorption, and influence on other panes in the insulating glass unit. The below table shows the solar energy absorptance, again as per EN 410:2011, for unit configurations with two 6 mm panes, with and without coatings.

 

6-(16)-6 Construction with Coating

Solar Energy Absorptance (%)

Outer Pane

Inner Pane

None

7.9

5.7

SGG PLANITHERM ONE T

(Surface 3)

10.4

9.2

SGG COOL-LITE SKN 176 II

(Surface 2)

30.0

1.3

PVB interlayers, included in our SGG STADIP range, even non-coloured, will increase the absorption of solar energy within a glass pane.

Glass Type

Solar Energy Absorptance (%)

8 mm SGG PLANICLEAR

9.6

8.4 mm SGG STADIP

16.8

9.5 mm SGG STADIP

21.7

A higher absorption of solar energy will result in a greater potential for a thermal differential in the glass.

EXTERNAL SHADING

The temperature differences generated can be increased due to the presence of shading, which as well as blocking incident solar radiation from portions of the glass; can also block incident solar radiation from heating up the framing system. As such under some conditions, any temperature increase in the frame will lag behind the heating of the glass due to solar radiation, resulting in a more significant thermal gradient and increased resultant stresses.

Backups & Blinds

Blinds will reflect solar energy back into glazing, absorb solar energy and heat the surrounding area and, if no ventilation is allowed, cause a heat trap between the blinds and the glazing, resulting in a localised temperature rise. Backups have a similar, but typically more severe effect.

When considering the internal conditions behind glazing, consideration should be given to allowing airflow between any obstacles, whether blinds or bulkheads, to reduce localised temperature rises.

INTERNAL HEATING

Internal heating, whether from trench heaters, air conditioning units or radiators, can create localised temperature increases near the glazing, and need to be considered when assessing thermal safety.

Radiant heaters need additional consideration due to the direct radiant heating of the glass, which will typically generate more sever temperature increases compared with warm air heating in the vicinity.

Thermal Safety Assessments

Requests for thermal safety assessments can be submitted through the Saint-Gobain Building Glass Specification Managers or online through the Thermal Safety Check Form.